Investigations
1st investigations to order
FBC
Test
Useful markers of rickettsial infection, but not specific.
Lymphopenia is usually mild and may be associated with abnormal lymphocyte morphology. This sign often leads to an erroneous diagnosis of a viral infection.
Thrombocytopenia may be mild or severe but is usually present during acute rickettsial infections.
Neutrophil levels are usually normal, unlike in most systemic bacterial infections.
Result
thrombocytopenia and lymphopenia
LFTs
Test
Serum aspartate aminotransferase and alanine aminotransferase are usually raised but not as high as in viral hepatitis, being <200 U/L in the former rather than >500 U/L in the latter.
Other LFTs are not usually raised and the patient is not jaundiced.
Result
elevated
serum electrolyte levels
Test
Useful marker of early rickettsial infection, but not specific.
Hyponatraemia is often seen at the height of the infection.
Result
hyponatraemia
CRP
Test
Useful marker of rickettsial infection, but not specific for rickettsial infection.
If >100 mg/L this would favour a diagnosis of rickettsial infection rather than a viral infection (CRP <50 mg/L).
Result
raised (>100 mg/L)
serology
Test
Very specific for rickettsial infection. However, may be negative early (<1 week) in the infection. Seroconversion or a significant rise in rickettsial antibody titres between two sera a few days apart confirms the diagnosis. Acute serum and convalescent serum are both tested together in the same laboratory assay. The old-fashioned Weil-Felix serological assay is no longer recommended for diagnosing rickettsial infections due to its poor sensitivity and poor specificity.
Result
antibodies present to Rickettsia species or Orientia tsutsugamushi
polymerase chain reaction (PCR)
Test
This is best done on a swab or biopsy of the eschar (if present). It will still be positive several days after antibiotics have been started. If done on blood it may give a false-negative result during the first several days of illness .[25] This assay is not widely available. It should be sent to a reference laboratory.
Result
a positive PCR is diagnostic of rickettsial infection
Investigations to consider
chest x-ray
Test
Only order if clinically indicated (e.g., pneumonia suspected). May be positive in murine typhus (Rickettsia typhi) and scrub typhus (Orientia tsutsugamushi).
Result
new infiltrates consistent with pneumonia
trial of doxycycline therapy
Test
All rickettsiae respond to doxycycline, so a therapeutic trial is often useful, although it is not a specific intervention as other infections will also respond well to treatment with doxycycline.
Result
rapid (within 48 hours) clinical improvement
cell cultures
Test
Can be performed on blood samples (during the early rickettsaemic phase) or from an eschar biopsy. These samples must be sent to a reference laboratory for inoculation of cell cultures.[25]
Result
growth of rickettsia
immunohistochemistry
Test
Can be performed on an eschar biopsy specimen.[25] This sample must be sent to a reference laboratory.
Result
detection of specific anti-rickettsial antisera
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